[ntp:questions] Re: Audio refclock+linux+soundcard = yuck

Ulrich Windl Ulrich.Windl at RZ.Uni-Regensburg.DE
Mon Sep 29 10:20:05 UTC 2003

shoppa at trailing-edge.com (Tim Shoppa) writes:

> 3.  A PCI sound card I bought a couple years back for $15 or so.  With
> both OSS and ALSA, it seems to start working pretty well.  In a couple
> of minutes it is syncing to the minute pulse, and it starts decoding
> digits in the timecode.  After another 10 or 15 minutes I'm getting all
> the digits.  BUT... the offset is circa 600 milliseconds!  Now a
> couple of those milliseconds come from WWV/H propogation and a few
> more probably from the radio's DSP filtering.  And the jitter is in
> the 100 millisecond range.
> Some more investigation with the #3 setup above shows that if I loop
> the sound from line in, through the linux drivers, and back out the
> line out, there is an audible most-of-a-second delay.  Undoubtedly
> NTP's measurement of 600 milliseconds is this same number.  Doing the
> same with setup #2 the delay is at most a millisecond or so.  (That
> took a scope to measure).
Maybe see /usr/src/linux/ for the file MAINTAINERS or CREDITS. Then
try to get in contact with some persons. Maybe they have some good
advice. Usually sound cards have buffers, and to unload the CPU, an
interrupt is only created if that buffer is mostly full (or
empty). Maybe you can control the threshold...


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